1. Field of the Invention
The present invention generally relates to an apparatus to generate a white underbase for printing of a given image on a colored background. More particularly, the present invention relates to an apparatus for generating white underbase to enable automatic printing of a given image on a colored background. The invention further relates to a method for generating white underbase to enable automatic printing of given images on colored substrate.
2. Description of Related Art
In general, printing is done on white substrate using process colors viz. CMYK with or without extended inks/spot inks like orange, blue, green, red etc. There are some applications where only the spot colors from the image are used. However, in the majority of the circumstances the printing plates are produced with white background in mind.
Still, there are certain applications where printing is required to be done on colored backgrounds including black. For such situations, a white underbase is used. Currently the methods deployed in the industry to meet the needs of such situations are inadequate and cumbersome, and are dependent upon the skill level of the operator producing the separations including the white underbase for making the plates for a printing process. Presently, almost all the separation work is done on computers and similar electronic devices. Hence these separations are used directly by the printing device without generating a plate for printing. Such a printing device could be a laser printer or an inkjet printer etc.
There are four widely used existing methods to generate the white underbase. They are as under:                1. The most common method used when printing over the black background comprises use of the black color of the background to replace the use of black ink in the process color printing. The white underbase so developed is a grayscale image having tints varying in accordance with the image to be printed. Currently the most common method used in the industry is to produce a negative of the image to be reproduced, convert it into gray image and then adjust the gray levels of the resulting image with the help of the operator. The amount of white to be present is actually decided on the basis of the wisdom of the operator. If the operator is experienced enough then a good starting point can be achieved that can be perfected in one or two further iteration. Otherwise it takes additional iterations of generating the white underbase including printing over the underbase before the right underbase setting can be achieved.                    More often than not, the operator ends up defining much of the area as solid white resulting in white more than necessary, thus resulting in inferior prints. Further, such a method involves an inherent weakness in the assumption that the grayscale negative of the image to be printed will define the white underbase properly. This assumption fails in almost all cases except when the image itself is a grayscale image or where image colors are light shades. This method for example, fails in case of dark and saturated colors except for few hues of yellow color. This method fails in case of the bright red or a bright blue colors as such colors will result in a dark gray value and hence very little amount of white ink will be used as the underbase. But printing of such colors indeed needs almost 100% white underbase as they do not contain any black component. Reference [1], [2] and [4].                        2. The second method used by the industry is to put solid white underbase every where wherever any color in the image is to be printed. This method is generally used in case of colored background, though the same can be used for black background as well. The printing is then done normally on the white underbase.                    This method fails if the image to be printed is having soft edges or vignettes for smooth blending with the background. The reason is that a solid white is printed as an underbase and the printing of the image is done over it assuming as if it is being printed now on the white background. The edges of the image where soft blending with the background should otherwise be there, become hard because of a sudden jump from the background color to the white color. Reference [1], [2], [3], [4], [5].                        3. In one method described in reference [4], it requires the use of two different production of the same image, namely on the black background and on the white background to be fed to the system to be able to create a white underbase with the help of the operator interaction.        4. There is another method disclosed in the reference [6], where the operator generates the white underbase interactively prior to printing of an image by selecting the color that is to be considered as the background with an option to soften the edges of the white underbase so generated.        
In all of the above prior art, the white underbase is generated by the operator, either interactively while giving the job for printing or is done offline in any image editing software and saved as a channel data for the white underbase [5] along with the image to be used for printing.